A double data rate synchronous dynamic random access memory (DDR-SDRAM) is known. The DDR-SDRAM is connected to a semiconductor device provided with a dedicated interface circuit. Data is communicated between the DDR-SDRAM and the interface circuit.
When a read operation is performed in the DDR-SDRAM, a data strobe signal and a data signal synchronized with an edge of the data strobe signal DQS are outputted from the DDR-SDRAM to the interface circuit through a plurality of transmission lines. The interface circuit connected to the DDR-SDRAM detects the edge of the data strobe signal in order to receive the data signal.
When a write operation is performed in the DDR-SDRAM, the data strobe signal is outputted from the interface circuit to the DDR-SDRAM. As described above, since the data strobe signal is a bi-directional signal, a transmission line of the data strobe signal enters into a high impedance state when the read and write operations are not performed.
When a read command is issued, the data strobe signal is changed from the high impedance state to a data strobe output state after a preamble period. When the transmission line of the data strobe signal enters into the high impedance state, pulse-shaped noise may easily occur in the data strobe signal due to the influence of crosstalk etc. When such noise occurs before an original data strobe is outputted, the interface circuit may receive a data signal incorrectly.
Conventionally, in order to prevent receiving a data signal incorrectly, the interface circuit is provided with a gate signal for setting an available period of the data strobe signal according to a latency of the DDR-SDRAM. The interface circuit receives the data signal using a data strobe signal gated by the gate signal.
However, due to variations of an operation voltage and a temperature of the DDR-SDRAM, the phases of the data strobe signal and the gate signal may change, or the transfer time of the data strobe signal which is transmitted through a transmission path may vary. In a conventional system, variation of a phase relation may occur between the data strobe signal and the gate signal due to such change or variation.
In such a case, when the above mentioned noise is included in a gated data strobe signal or a primary data strobe is not outputted, a data signal may be received incorrectly. Especially, in high speed transmission beyond 1 Gbps, the phase change amount becomes so large that it can not be ignored.